Sensor and method for detecting analytes in fluids

ABSTRACT

A fluid sensor is constructed to has a pair of electrodes whereas between electrodes there are not additional materials designated to adsorb analytes if their concentrations are high, or there are absorbents if the analyte concentrations are low. An alternating current voltage of varying frequencies is applied to the electrodes of the sensor by an alternative current device. In return, it detects electrical properties such as impedance and its components, reactance, resistance, and phase angles of the sensor with analytes whereas the analytes reside in or pass through the electrode at each frequency. Thus a spectrum of electrical property of the analyte can be established at various applied frequencies. The electrical properties are analyzed by a pattern recognition process, and compared with those of the known fluid. Therefore, the fluid can be detected and identified. A reference sensor is provided with the same configuration of the fluid sensor. With combining electrical signals from the fluid sensor and reference sensor, the present invention brings a number of advantages, including elimination of humidity influence, polymer film aging effect, and effect of temperature variation. In addition, a temperature programming is suggested in the present invention be better control processes of adsorption and desorption for analytes thus the analytes can be better detected and identified.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of sensors, andmore particularly relates to sensors for detecting analytes in fluids.

2. Description of the Prior Art

Sensors are widely used in technology of detecting the presence ofanalytes in fluids. The following references are pertinent to this fieldof art:

-   -   1. U.S. Pat. No. 4,887,455 issued on Dec. 19, 1989 to Payne et        al. for “Gas Sensor” (hereafter “the Payne '455 patent”);    -   2. U.S. Pat. No. 5,571,401 issued on Nov. 5, 1996 to Lewis et        al. for “Sensor Arrays for Detecting Analytes in Fluids”        (hereafter “the '401 Lewis patent”);    -   3. U.S. Pat. No. 6,319,724 issued on Nov. 20, 2001 to Lewis et        al. for “Trace Level Detection of Analytes Using Artificial        Olfactometry” (hereafter “the '724 Lewis patent”); and    -   4. Payne, et al., “High-Frequency Measurements of Conducting        Polymers: Development of A New Technique for Sensing Volatile        Chemicals”, Meas. Sci. Technol. 6 (1995) pp. 1500-1507        (hereafter “the Payne Publication”).

The Payne patent discloses a gas sensor that has a semiconductor organicpolymer layer exposed to a gas to be detected. An analyzer applies analternating electric signal at specific resonant frequencies to thesensor to detect the change in the sensor's impedance characteristicswhich are compared by a microcomputer with reference characteristicsstored in a memory of the microcomputer. The gas in contact with thesensor can be detected because of the resulting difference spectra. Thepatent further discloses that the best performance of the invention islikely to be conducted between frequency ranging 100 MHz to 500 MHzwherein the resonance may happen.

The '401 Lewis patent discloses arrays of chemical sensors for detectinganalytes in fluids. The sensors include first and second conductiveelements electrically coupled to and separated by a chemically sensitiveresistor which provides an electrical path between the conductiveelements. The resistor includes a plurality of alternating nonconductiveregions made of a nonconductive organic polymer and conductive regionsmade of a conductive material transverse to the electrical path. Theresistor further provides a difference in resistance between theconductive elements when contacted with a fluid containing a chemicalanalyte at a first concentration, and then at a second differentconcentration. Arrays of such sensors are constructed with at least twosensors having different chemically sensitive resistors providingdissimilar such differences in resistance. Variability in chemicalsensitivity from sensor to sensor is provided by qualitatively orquantitatively varying the composition of the conductive and/ornonconductive regions. An “electronic nose” for detecting an analyte ina fluid may be constructed by using such arrays in conjunction with anelectrical measuring device electrically connected to the conductiveelements of each sensor.

The '724 Lewis patent discloses a method using artificial olfactometryfor detecting the presence of an analyte indicative of various medicalconditions, including halitosis, periodontal disease and other diseases.

The Payne Publication discloses the change in the alternate current (AC)impedance characteristics of poly-N-(2-pyridyl) pyrrole in the presenceof different volatile chemicals.

It can be seen from the above cited references that significant effortshave been devoted in the past in the research and development of sensorsthat are capable for detecting and identifying analytes in fluids.Identification of analytes in fluids from instrumental analysis isaccomplished from mimic mechanisms of the mammalian olfactory systemthat applies probabilistic repertoires of many different receptors torecord a single odorant.

However, identification of the odorant is dependent upon not only theresults from highly specific receptors but also the output from lessspecific ones. In other words, identification is based on recognition aspectrum of signals that resemble a specific pattern. Following thisdirection, conventional technologies in sensor configuration weredeveloped according to the following two schemes to generate a signalspectrum: applying a multiple sensor and single sensor strategies.

In the approaches that utilize multiple sensors, various detectingdevices have been developed that use metal oxide thin film resistorsensors, conductive polymer or polymer carbon powder composite filmchemi-resistor sensors, polymer coated quartz crystal microbalance (QCM)sensors, polymer coated surface acoustic wave (SAW) sensors,metal-oxide-silicon field-effect-transistor (MOSFET) sensors, andoptical sensors. However, although much progress has been made in thepast, there are still primary disadvantages inherited from the sensingmechanisms of such multi-sensor technologies. The disadvantages includethe requirement of a large number of sensors to generate a patternedinformation, the sophistication required for the sensor configuration,the poor reproducibility in sensor manufacturing, the strong humidityinfluence on chemical analysis, the slow response, the expensiveelectronics equipment required, and the very restricted operatingconditions.

Various polymer films with a general thickness of several micrometershave been extensively used in multi-sensor configurations to improvesensor sensitivity and detection limit. This is primarily due to thefact that the polymer films can trap the chemical vapor because of theirspecific chemical selectivity on analytes. As a result, the analyteswill be concentrated inside of the polymer prior to detection.

However, the conventional polymer films also inherit a number ofdisadvantages. First, the thin films of polymer are sensitive to thehumidity associated with the analyte. Humidity is the predominant factorto influence performance of the polymer film based gas sensors. Second,polymer films have an aging effect that affects the sensor stability forlong term operations. Third, it is difficult to achieve reproducibilityof dispensing the polymer films onto sensors, particularly when a largenumber of sensors must be used in a multi-sensor configuration.

In the approaches that utilize a single sensor strategy, variousinstruments have been developed that are based on the mechanisms of gaschromatography (GC), mass spectrometry (MS), and light spectrum.Generally, these instruments are very expensive. Moreover, they aretypically very bulky in sizes that makes miniaturization almostimpossible. As a result, they are less attractive in the market whereportability of instrument becomes increasingly important.

As an example, the Payne patent and Publication discussed above discloseapplication of a single sensor for detecting impedance and phasesensitive components of conductive polymer modified electrodes atvarious frequencies to establish a spectrum of signals. However, thePayne device requires high frequencies ranging from 100 MHz to 500 MHz,which brings significant difficulties in instrument manufacturing andapplication. In addition, it still has the disadvantages inherent frompolymer films.

Therefore, it is desirable to design and develop a new sensor and methodthat overcome the disadvantages of conventional sensor devices, and hasa better reproducibility of performance and sensor manufacturing, fewerinterference deficiency, enhanced sensitivity, less restricted operationconditions, and increased portability.

SUMMARY OF THE INVENTION

The present invention is directed to a sensor and related method fordetecting analytes in fluids.

It is an object of the present invention to provide a new and uniquesensor device and technique for detecting analytes in fluids thatutilize a single sensor design for analyzing AC electrical informationof the analytes at various selected frequencies ranging from 10 KHz to 1MHz.

It is also an object of the present invention to provide a new andunique sensor device and technique for detecting analytes in fluids thatcan identify an analyte by comparing a single pattern of the ACelectrical information of the analyte at various selected frequenciesranging from 10 KHz to 1 MHz with patterns of electrical information ofknown analytes.

It is another object of the present invention to provide a new andunique sensor device and technique for detecting analytes in fluids thathas a background reference mechanism to reduce the background influence,including humidity, on the analyte detection.

It is a further object of the present invention to provide a new andunique sensor device and technique for detecting analytes in fluids thatemploys temperature programming on regulating the sensor temperature toimprove the performance of the analyte detection.

It is the further object of the present invention to provide a new andunique sensor device and technique for detecting analytes in fluids thathas capability for applying all types of organic, inorganic, and metaladsorbents to selectively analyze analytes including small moleculechemicals.

It is an additional object of the present invention to provide a new andunique sensor device for detecting analytes in fluids that is compact insize, portable, easy to use, inexpensive to produce, and low in energyconsumption.

In a preferred embodiment of the present invention, a novel and uniquedetection method and device is provided for identifying analytes influid, which is based on detecting AC electrical properties of theanalytes such as impedance and its components, i.e., resistance andreactance, as they are governed by chemical characteristics of theanalytes. The present invention utilizes a single sensor that has a pairof metal conductors to analyze AC electrical information of the analytesat various frequencies ranging from 10 KHz to 1 MHz.

The measurement results in a patterned electrical information for theanalytes at the applied frequencies. After analyzing the AC electricinformation at various selected frequencies through a patternrecognition procedure, the analytes can be identified by comparison withpatterns of electrical information of known analytes.

In the present invention method, the identification of chemicals isbased on generating patterns of characteristic information gathered fromthe chemicals in the analytes being tested, which information arespecific to the chemicals and are gathered from various dimensions.Therefore, the present invention method is focused on finding suchspecific characteristics information that is related to the naturalcharacteristics of the chemicals. For example, dielectric constant isone of the natural characteristics of chemicals, and can be used toidentify chemicals. The dielectric constant can be measured electricallyby the capacitive reactance in impedance complex in the vector domain.Hence capacitive reactance is one of the characteristic informationuseful for identifying chemicals (in the scalar domain, capacitance isproportional to the dielectric constant and has been used foridentifying chemicals).

In addition, each chemical has its unique composition of chemicalelements which results in specific chemical characteristics. Suchcharacteristics may be measured by resistance. For example, volatileorganic chemical rich in hydrogen and oxygen changes electricconductivity (resistivity) of metal oxide based sensors.

Since chemicals in pure form (including their gas phase) are notelectrically conductive, capacitive reactance is the dominant componentin impedance comparing with resistance. However, although resistancevalue is small, it reflects resistive characteristics of chemicals anddescribes their properties in another dimension. Therefore, resistanceinformation is also important for identifying chemicals.

Impedance can be obtained applying Ohm's law in AC condition:Z=V/I  [1]where Z is the impedance vector, V is the voltage vector, and I is thecurrent vector. It can be understood that from the above equation, thevoltage across the sensor is proportional to the impedance under aconstant current technique. Current passing through the sensor electrodeis reverse proportional to the impedance applying a constant voltagetechnology. Therefore, as alternatives, either current or voltage may beused (in the place of impedance) for identification purpose.

The present invention sensor and method does not require additionalmaterials for adsorbing chemical vapors between the metal conductors inthe sensor design when the analytes are sufficiently concentrated. Whenthe analyte concentrations are low, the present invention sensor andmethod has the option of using adsorbent materials in the sensor designto improve the sensitivity and detection limit of the sensor.

The present invention has many novel and unique features and advantages.In summary, the present invention sensor and method for detectinganalytes in fluids utilize a single sensor design operating within afrequency range of under 1 MHz which results in an easy instrumentdesign and application. It does not require the use of a conductivepolymer film in the sensor structure and has capability to apply alltypes of adsorbents. It further provides a background reference and hasa low energy consumption that allows the use of a temperatureprogramming to more precisely control the processes of adsorption anddesorption for analytes, which improves the detection and identificationof the analytes. In addition, the present invention sensor device has alow cost to manufacture and is compact in size which makes it portableand easy to use.

It is noted that one of the main advantages of the present invention isto use single sensor configuration to generate a spectrum of chemicalsignals. The reproducibility of manufacturing sensors can be easilyachieved with a single sensor strategy when a few pairs of identicalsensors are needed for an instrument. The small size which results inlow power consumption for controlling sensor temperature and smallvolume requirement also allow the implementation of a dual sensingdetection strategy which, in addition to an analytical sensor,incorporates a reference sensor with the identical configurations asthat of the analytical sensor.

With this dual sensors configuration, samples of an analyte withbackground subjects such as humidity levels are tested by the analyticalsensor while only the background subjects is are tested by the referencesensor. By comparing the outputs from the analytical and referencesensors, the background effect can be removed from the test result ofthe sample analyte. Similarly, the aging effect for polymer film basedsensors, and changes in testing responses induced by temperaturevariation, can also be removed or eliminated.

These and further novel features and objects of the present inventionwill become more apparent from the following detailed description,discussion and the appended claims, taken in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring particularly to the drawings for the purpose of illustrationonly and not limitation, there is illustrated:

FIG. 1 is an illustrative plot diagram showing the result ofclassification for five (5) chemicals from a principal componentanalysis, where resistance and reactance at seven (7) frequencies withina range of 10 KHz to 500 KHz are used in the analysis;

FIG. 2 is an illustrative plot diagram showing the result ofclassification for five (5) chemicals from the principal componentanalysis, where resistance and reactance at eleven (11) frequencieswithin a range of 500 KHz to 1000 KHz are used in the analysis

FIG. 3 illustrates a schematic diagram of a single sensor 1 containing apair of metal wires or plates acting as electrodes or capacitors;

FIG. 4A shows a dual sensor configuration which utilizes two identicalsensors 1, one as an analytical sensor 2 (AS in abbreviation), and theother as a reference sensor 3 (RS). FIG. 4A shows the dual sensorconfiguration 12 without an integrated form of two sensors;

FIG. 4B shows a dual sensor configuration which utilizes two identicalsensors, one as an analytical sensor 2, and the other as a referencesensor 3. FIG. 4B shows an integrated form 13;

FIG. 5 shows a diagram for electrically connecting a single sensor 1 toan alternating current analyzing device 9;

FIG. 6 shows a schematic of any type of adsorbent 8 placed ontoelectrodes or capacitors for adsorbing analytes from a fluid;

FIG. 7A shows a schematic diagram to show a different configuration 4 toform a single sensor where an electrode acting as a first electrode, andelectrically conductive structure member, such as a sidewall acting as asecond electrode; and

FIG. 7B shows a sidewall is properly grounded in connection of thesensor 4 to the alternating current analyzing device 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although specific embodiments of the present invention will now bedescribed with reference to the drawings, it should be understood thatsuch embodiments are by way of example only and merely illustrative ofbut a small number of the many possible specific embodiments which canrepresent applications of the principles of the present invention.Various changes and modifications obvious to one skilled in the art towhich the present invention pertains are deemed to be within the spirit,scope and contemplation of the present invention as further defined inthe appended claims.

Referring to FIGS. 3, 4A, 4B and 5, the present invention sensor andmethod for detecting and identifying analytes in fluids have two mainobjectives. Referring to FIG. 3, the first major objective of thepresent invention is to use a single sensor 1 configuration foreffective identification of analyte by detecting certain electricalproperties associated with their distinguished physical and chemicalcharacteristics, such as dielectric constant, element electronegativity,and polarity. This first main objective of the present invention isachieved by applying an impedance spectroscopy with frequency sweepingtechnique. With a frequency sweeping, a pattern of information isconstructed at various selected frequencies. By analyzing theinformation with a pattern recognition process, the analytes can beidentified in comparison with those of known substances. The second mainobjective of the present invention is to provide a method that caneliminate background subjects, such as humidity, which influence theanalysis of analytes. Referring to FIGS. 4A and 4B, the second mainobjective is achieved by applying a dual sensing configuration whichutilizes two identical sensors 1, one as an analytical sensor 2 and theother as a reference sensor 3.

It should be indicated that implementation of the dual sensing strategyis practically applicable in the present invention due to the advantageof small volume requirement from applying the single analytical sensorto obtain the patterned information for the analytes.

Gases and volatile chemicals have their distinguished dielectricconstants. They also contain various chemical elements that have theirdistinguished values of electronegativities. In addition, in terms ofmolecular symmetry, gases and volatile chemicals have their uniquemolecular sizes and shapes. These factors determine characteristics forchemical analytes in their adsorption and desorption processes.

These physical and chemical characteristics can be described withcertain AC electric properties, such as impedance (Z) and its phasesensitive components: reactance (X), resistance ®, and phase angle (θ),as the analyte is under an AC voltage excitation. The AC electricproperties can be obtained by using an instrument of impedance analyzeror other known alternating current devices 9. In its simplest form, atest device can be a pair of metal wires or plates acting as electrodesor capacitors. The AC electric properties of the gases or chemicalvapors are obtained as gases or chemical vapors reside or pass throughthe electrodes or capacitor.

Impedance is described asZ=R+X  [2]in complex (with the bold letters indicating vectors). The phase angle(θ) can be calculated from the R and X values.

Reactance X of the sensor electrode in the present invention is combinedwith a reactive capacitance X_(C), and reactive inductance X_(L), whichcan be described as follows:X _(C) =j(−1/2πfC)  [3]X _(L) =j(2πfL)  [4]where C is a capacitance that is proportional to a dielectric constantof the medium residing between electrodes, and L is the inductance ofthe electrode. For a small electrode, the value of inductance L is smalland the reactance X is predominantly capacitive.

If in the absence of analytes the reactive capacitance is X_(C)(1), andin the presence of analyte the reactive capacitance is X_(C)(2), thentheir difference ΔX_(C):ΔX _(C) =X _(C)(2)−X _(C)(1)  [5]can be obtained, which is the change of reactive capacitance as theresult of the presence of the analyte.

Since the capacitance of analytes is determined by the their dielectricconstant, reactive capacitance can be used to detect and identify theanalytes. This means reactive capacitance provides a signatureinformation of each chemical. By varying the frequencies, the presentinvention is able to construct a reactance spectrum to record electricalproperty of the analytes at each frequency. This results in obtainingpatterned chemical characteristics of the analytes.

Gases and chemical vapors will be adsorbed by the surface of the testdevice. This capability creates a complicated diffusive process, andsurface interfacial kinetics or surface resistance for the analyte,which are associated with their distinguished molecular characteristics.For example, exposure to hydrogen or oxygen-rich of volatile organiccompounds noticeably changes the electrical conductivity of metal oxidesensors.

Since oxygen or hydrogen has its defined electronegativity, the changingof conductivity (resistivity) indicates that resistance can be used torecord chemical characteristics of vapors.

In the present invention, a series of resistance information is alsogenerated with varying frequencies. The change of resistance can bedefined as:ΔR=R(2)−R(1)  [6]where R(1) is the resistance of electrode substrate in the absence ofany analytes and R(2) is the resistance of substrate exposed to theanalytes.

Comparing magnitude of resistance and reactance, it is noted thatreactance is the predominant factor to govern values of impedance sincechemicals are not conducting at their dry phase. It is also noted thatphase angles (θ) can be calculated from R and X. Therefore, the changeof phase angle (θ) is also readily available. Combining the informationof change of reactive capacitance and resistance and/or change of phaseangles, a matrix of electrical property is constructed that containschemical characteristics of the analytes at various selectedfrequencies.

As the change of electrical property varies nonlinearly with frequency,the variation of the electrical property change can be analyzed througha pattern recognition process including applications of multivariateanalysis method. Applying such pattern recognition analysis on thematrix of electrical property, the chemical characteristics can beidentified.

As a result, the analyte can be differentiated by comparing thepatterned chemical characteristics of the analyte with those of knownsubstance.

The preferred frequency range of the present invention method is from 10KHz to 1 MHz. Applying the various selected frequencies can not onlyidentify the analytes but also increases the options of instrumentdesign and practical application.

The present invention sensor design does not require the use of anyconductive polymer film for identifying analytes at high concentration.This is because the AC voltages can be applied across the vacuum betweenthe two electrodes without conductive materials placed in between. Thisis Well suitable to be used as a detection method of gas chromatographywhere the analyte vapor is separated and concentrated in thechromatographic process. This new detection method will provideinformation on not only the quantity but also the identity of theanalytes in gas chromatography.

In addition, referring to FIG. 6, since AC signal can travel throughvacuum, the present invention can use any type of adsorbent 8 whether ornot they are electrically conductive for detecting analytes toconcentrate or selectively concentrate dilute vapor. Therefore, thepresent invention method can utilize a variety of developed techniqueson concentrating chemicals or selectively concentrating marker gases ofanalytes in fluid for dilute analytes to improve sensitivity andselectivity of the sensors. Such techniques typically involve polymerfilms, polymer inorganic materials composites, composites containingpalladium, solid inorganic materials used as stationary phase inadsorption chromatography, and polymeric materials used as stationaryphase in preparation of packed column in gas partition chromatography.

For example, hydrogen is an important industrial gas for manyapplications. It is often critical to detect and identify the hydrogengas due to safety concerns. In the present invention, the hydrogen gascan be selectively concentrated by applying composites containingpalladium particles, and organic or inorganic fillers. This is becausethe hydrogen gas has a large solubility in the palladium metal (oftenreferred to as a “hydrogen sinker”). Since the present invention analytedetection method is not limited to adsorbent materials that areelectrically conductive, it has a great flexibility to choose anypercentage of palladium particles in the composite to meet theconditions of required hydrogen gas concentration in analytes for theapplication of the present invention.

Furthermore, adsorption chromatography uses solid particles asstationary phase to selectively adsorb molecules. Various solidinorganic materials can be used for this purpose. Among them, MolecularSieves, silica gel, alumina, porous carbon particles, and calciumcarbonate are the most popular choices.

For example, Molecular Sieves retain its adsorbents, by strong physicalforces, and separate molecules based on their size, configuration,polarity, and degree of unsaturation. Molecular Sieves will adsorbcarbon monoxide in preference to argon. They preferentially adsorb polarmolecular molecules containing oxygen, sulfur, chlorine, or nitrogenatoms, and asymmetrical molecular molecules containing oxygen, sulfur,chlorine, or nitrogen atoms, and asymmetrical molecules. MolecularSieves also effectively trap ethylene or propylene from saturatedhydrocarbons.

In gas partition chromatography (GC), separation of chemicals in mixtureis based on vapor pressure of chemicals and selective interactionsbetween chemicals and polymeric materials used as the stationary phaseof the GC columns. The stationary phase materials are coated onto solidsupport substance. At the molecular level, the interaction is based onintermolecular forces between chemical and materials of the stationaryphase such as dispersion, induction, orientation, and donor-acceptorinteraction. To help the understanding of the processes described above,the term “like dissolves like” may be useful to explain results of suchintermolecular interactions. Selectivity is one of results of theinteraction due to a similarity between “likeness” of a kind ofchemicals and materials of the stationary phase of the column. Forexample, polarity is a physical parameter of chemicals and can be usedto describe such likeness. Polar chemicals like polar materials of thestationary phase, and non polar chemicals go to non polar stationaryphase.

Based on this principle, objective of selective partition of a type ofchemicals can be achieved by using a kind of stationary phase materialswhose polarities are close or match the polarities of the chemicals. Forexample, polysiloxane is a non polar material. It is most popularly usedas the base materials of the GC stationary phase since its basicchemical structure can be readily derived by methyl, vinyl, phenyl,diphenyl,3,3,3,-trifluoropropyl, 2-cyanoethyl, or 3-cyanopropylconstituents to change its polarity from non polar to polar. Therefore,specifically derived polysiloxane polymers are appropriate to many typesof industrial chemicals in terms of closeness of their polarities.

The following polymers are often involved in applications for theindustrial chemicals in fluid:

1. Poly(100% dimethylsiloxane) for analytes of solvents, petroleumproducts, fuel, oil, hydrocarbons, pharmaceuticals, flavors, andfragrances.

2. Polymers containing (5% diphenyl/95% dimethyl), or (35% diphenyl/65%dimentyl), or (14% cyanopropyl/86% dimethyl) for analytes of pesticides,aromatic hydrocarbons, polychlorinated biphenyl, oxygenates, amines,essential oil, pharmaceuticals, environmental samples, and nitrogencontaining herbicides.

3. Poly(20% diphenyl/80% dimethylsiloxane) for flavor aromatics andalcoholic beverage.

4. Polymers containing (50% phenyl/50% methyl) or(trifluoropropylmethyl) for environmental chemicals, solvents, andketones.

5. Ploy(65% diphenyl/35% dimethylsiloxane) for analytes of phenols andfatty acids.

6. Poly(50% cyanopropylmethyl/50% phenylmethylsiloxane) for analytes ofcarbohydrates and neutral sterols.

The derived polysiloxane polymers can be coated onto solid supportparticles for preparation of packed columns. The solid support can beporous ceramics including alumina, silica, and glass, or otherparticles.

Applying a single sensor with electric frequency sweeping technique, thepresent invention is able to not only obtain a patterned information ofanalyte but also gain a distinctive advantage of having small size ofsensor and sensor compartment in sensor design. The small size of sensorand its compartment make the present invention practically be able toutilize a dual sensing detection in sensor manufacturing, particularlyfor manufacturing handheld and battery powered electronic noseinstrument.

The present invention applies two identical sensors one serving as ananalytical sensor, and the other as a reference sensor. When backgroundsubjects such as humidity is a critical factor that affects sensorperformance, the analytes with background subjects are tested with theanalytical sensor, but only the background subjects are tested with thereference sensor. By subtracting electrical property of the analyticalsensor from that of the reference sensor, the influence of backgroundsubjects, including humidity, can be eliminated.

One application of the this dual sensor arrangement is for in situmedical diagnostic applications, for example detecting ear and mouthdiseases for patients. In such applications, the analytes of chemicalvapor generated by bacteria caused by diseases are overlapped by thehumidity in breath where the water concentration in humidity issignificantly higher than that of the analytes. By using a referencesensor, the humidity can be measured by the reference sensor and itselectrical property can be subtracted from the electrical properties ofthe analytical sensor which measures both the analytes and the humiditylevel. As a result, the information of the analytes can be obtainedwithout the error introduced by the humidity level. Similarly, this dualsensor arrangement of the present invention can also be used tocompensate the polymer film aging effect, or influence caused bytemperature variations.

Gases and chemical vapors will be influenced by temperature in theiradsorption and desorption process. In gas chromatography, a temperatureprogramming process is often applied for efficient separation ofchemical mixture. Because of its small size of the sensor and sensorcompartment which result in low power consumption for temperatureregulation on sensor and analyte, the present invention can utilizetemperature programming to control adsorption and desorption process ongases and chemical vapors in their interaction with sensor electrodes.This makes it possible that chemical characteristics of the analytes canbe fully explored by the present invention method.

The present invention enables the design and development of sensorinstrument with a small size, low cost and great portability. As anexample, the present invention sensor is well suitable for a disposableelectrode configuration in design of electronic nose instrument.

EXAMPLES

The following are examples and experimental information of the presentinvention sensor and method which are offered by way of illustrationonly and not by way of limitation.

A pair of electrodes were constructed with gold wires. The electrodeswere 12 mm in length and had a gap up to 1 mm. The electrodes wereconnected to an instrument of impedance analyzer, such as an “Agilient4294A” analyzer. Calibration of electrodes was proceeded prior to samplemeasurement. A frequency sweeping method was used in impedancemeasurement where resistance and reactance or impedance and phase anglecould be simultaneously obtained at various frequencies within the sweptfrequency range.

Five chemicals were used in impedance tests, including acetone, aceticacid, hexane, toluene, and water. In the experiment, each chemical wasalternatively measured six times. In doing so, each chemical was filleda half full into six vials, which were tightly sealed except for sampletesting.

Before measuring each chemical sample, room air was first measured andrecorded, and its impedance, resistance, and reactance were used asreferences. To measure a sample, a vial containing such sample wasunsealed and placed where the liquid surface was close to the electrode.A cotton ball was used to block a joint area of electrode cable and vialopening to prevent variation of chemical vapor concentration inside thevial. Then a waiting period of ten seconds were applied before takingthe impedance data. After the measurement was done, the vial wasimmediately taken away from the electrodes and resealed. The measuredvial was not reused.

The electrodes were then exposed to room air again for about ten minutesbefore the next measurement. The sequence of measuring chemicals was inthe order of toluene, acetone, hexane, water, and acetic acid. Thesecond and subsequent (up to the sixth) measurements were taken with thesame sequence. Change of electrical property is obtained from thefollowing equation:Change of electrical property=Electrical property of sample−Electricalproperty of air  [7]Thus a raw data matrix was constructed from measuring each chemical atvarious frequencies, wherein each two columns represented resistancechange and reactance change at one frequency, respectively, and each rowrepresented a single measurement of a chemical.

Referring to FIG. 1, there is shown the principal component analysis(PCA) for chemicals of acetic acid, acetone, hexane, toluene, and waterwith their resistance and reactance data obtained at 10, 20, 50, 100,200, 300 and 500 KHz. The electrical information of the chemicals issimplified after applying principal component analysis and presented inaccordance with two principal components F1 and F2. During data analysisapplying PCA, the raw data of change of resistance and reactance in thematrix was autoscaled and normalized to the length one before furtherprocessing. It is clear from the graph that the test results from thesame chemical are grouped in a particular area, as the 5 chemicals areseparated and located in different areas of the F1 and F2 plane. Theresults of principal component plotting indicate that chemicals can bedistinguished with their electrical properties, such as impedance andits phase sensitive components (i.e., resistance and reactance) obtainedfrom frequency ranging from 10 KHz to 500 KHz.

Referring to FIG. 2, there is shown the results of separation from theprincipal component analysis for chemicals of acetic acid, acetone,hexane, toluene, and water, where their resistance and reactance datawere obtained at frequencies 502, 550, 600, 651, 700, 750, 801, 850,901, 949 and 1,000 KHz according to the present invention. Each chemicalwas repeatedly measured three times in accordance with the proceduredescribed above. During data analysis applying PCA, the raw data matrixof change of resistance and reactance was autoscaled and normalized tothe length one before further processing. The results indicate thatimpedance and its components can be used to identify chemicals.

It is understood that the application of the above specified frequenciesis only for illustration of effectiveness for the present invention. Itis not intended here to limit other frequencies within the frequencyranges in applications.

Furthermore, it can be understood from the above experimental resultsthat, instead of identifying analytes in fluids from multiple frequencydetection, the present invention can also be used to detect presence ofanalytes of interests with a single frequency application. For example,in GC chemical analysis, chemicals are eluted separately out of thecolumn by a carrier gas (usually hydrogen, or helium, or nitrogen). Eachchemical can be detected in presence because of different AC electricalproperties from the carrier gas and chemicals at the applied frequency.As another example, detection of known flammable gases such as methaneis critical for safe operation in mine industry. Detection of such gasescan be achieved at a pre-selected frequency with application of coretechnologies in the present invention.

In addition, referring to FIGS. 7A and 7B, in the single sensoraccording to the present invention, the two electrodes may be formed inmany ways 4. For example, when a first electrode is positioned at alocation close to a sidewall of a container made of electricallyconductive material which is properly grounded, then the sidewall of thecontainer may serve as a second electrode. The fluid to be tested can bedirected to pass through the gap between the first electrode and thesidewall of the container which functions as the second electrode.

The present invention has many advantages. It utilizes a singleanalytical sensor to generate a spectrum of chemical signals. The smallsize which results in low power consumption on sensor temperatureregulation and small volume requirement also allow the implementation ofa dual sensing detection strategy which, in addition to the analyticalsensor, incorporates a reference sensor with identical configurations.With this dual sensors configuration, the effects of background subjectssuch as humidity levels, or other factors such as the aging effect forpolymer films and changes in testing responses induced by temperaturevariation, can also be eliminated.

The present invention sensor and method also operate within a lowfrequency range between 10 KHz and 1 MHZ. The low energy consumption ofthe present invention sensor further allows the use of temperatureprogramming to improve the detection and identification of the analytes.Applying the electric impedance method, the present invention providescapability to use all types of absorbents, including organic, inorganic,and metal materials. In addition, the present invention sensor device islow cost to produce, compact in size, portable, and easy to use.

Of course the present invention is not intended to be restricted to anyparticular form or arrangement, or any specific embodiment, or anyspecific use, disclosed herein, since the same may be modified invarious particulars or relations without departing from the spirit orscope of the claimed invention hereinabove shown and described of whichthe apparatus or method shown is intended only for illustration anddisclosure of an operative embodiment and not to show all of the variousforms or modifications in which this invention might be embodied oroperated.

The present invention has been described in considerable detail in orderto comply with the patent laws by providing full public disclosure of atleast one of its forms. However, such detailed description is notintended in any way to limit the broad features or principles of thepresent invention.

Although the above invention is described in some detail by way ofillustration and example for purposes of clarity of understanding, it isapparent to those of ordinary skill in the art in light of the teachingof this invention that many changes and modifications may be madethereto without departing from the scope of the appended claims.

1. A sensor, comprising: a. a single analytical sensor comprising twoelectrodes; b. said two electrodes are maintained in a closely spacedapart relationship; c. means for connecting said single analyticalsensor having said two electrodes to an alternating current (AC)analyzing device; d. a fluid containing analytes is positioned betweensaid two electrodes, said fluid containing said analytes bridging saidtwo electrodes; e. means for applying various frequencies of AC signalsto said fluid containing said analytes; f. means for detecting ACrelated electrical properties of said fluid containing said analytesaccording to said applied various frequencies of AC signals, saidelectrical properties are detected at each of said applied frequencies;and g. said AC related electrical properties according to a multiplicityof said applied frequencies are used for identification of said analytespresent in said fluid.
 2. The sensor as claimed in claim 1, wherein saidAC related electrical properties include impedance of said fluidcontaining said analytes.
 3. The sensor as claimed in claim 1, whereinsaid AC related electrical properties include resistance of said fluidcontaining said analytes.
 4. The sensor as claimed in claim 1, whereinsaid AC related electrical properties include reactance of said fluidcontaining said analytes.
 5. The sensor as claimed in claim 1, whereinsaid AC related electrical properties include phase angle of said fluidcontaining said analytes.
 6. The sensor as claimed in claim 1, whereinsaid AC related electrical properties include voltage of said fluidcontaining said analytes.
 7. The sensor as claimed in claim 1, whereinsaid AC related electrical properties include current of said fluidcontaining said analytes.
 8. The sensor as claimed in claim 1, whereinsaid electrodes are made of electrically conductive materials.
 9. Thesensor as claimed in claim 1, further comprising a layer of materialbetween said two electrodes for adsorbing said analytes in said fluid.10. The sensor as claimed in claim 9, wherein said layer of material isselected from the group consisting of polymer, polymer inorganicmaterial composite, Molecular Sieves, silica gel, alumina, porouscarbon, and calcium carbonate particles, composite of palladiumparticles and inorganic materials, or composite of palladium particlesand polymer, polymer modified porous ceramic particles including porousalumina, or polymer modified porous silica, and polymer modified porousglass beads.
 11. The sensor as claimed in claim 1, further comprisingmeans for implementing a temperature programming technique.
 12. Thesensor as claimed in claim 1, further comprising a reference sensoridentical to said single analytical sensor for measuring AC relatedelectrical properties of background environment accompanied with onlysaid fluid.
 13. The sensor as claimed in claim 12, wherein saidbackground environment includes the group comprising temperaturevariation, polymer film aging effect and humidity level.
 14. The sensoras claimed in claim 1, wherein said AC related electrical properties areused for detection of said analytes present in said fluid.
 15. Thesensor as claimed in claim 1, further comprising odorants as saidanalytes.
 16. A sensor, comprising: a. a single analytical sensorcomprising a first electrode and a structural member functioning as asecond electrode; b. said first electrode and said structural member aremaintained in a closely spaced apart relationship; c. means forconnecting said single analytical sensor having said first electrode andsaid structural member to an alternating current (AC) analyzing device;d. a fluid containing analytes is positioned between said firstelectrode and said structural member, said fluid containing saidanalytes bridging said first electrode and said structural member; e.means for applying various frequencies of AC signals to said fluidcontaining said analytes; f. means for detecting AC related electricalproperties of said fluid containing said analytes according to saidapplied various frequencies of AC signals, said electrical propertiesare detected at each of said applied frequencies; and g. said AC relatedelectrical properties according to a multiplicity of said appliedfrequencies are used for identification of said analytes present in saidfluid.
 17. The sensor as claimed in claim 16, wherein said AC relatedelectrical properties include impedance of said fluid containing saidanalytes.
 18. The sensor as claimed in claim 16, wherein said AC relatedelectrical properties include resistance of said fluid containing saidanalytes.
 19. The sensor as claimed in claim 16, wherein said AC relatedelectrical properties include reactance of said fluid containing saidanalytes.
 20. The sensor as claimed in claim 16, wherein said AC relatedelectrical properties include phase angle of said fluid containing saidanalytes.
 21. The sensor as claimed in claim 16, wherein said AC relatedelectrical properties include voltage of said fluid containing saidanalytes.
 22. The sensor as claimed in claim 16, wherein said AC relatedelectrical properties include current of said fluid containing saidanalytes.
 23. The sensor as claimed in claim 16, wherein said electrodesare made of electrically conductive materials.
 24. The sensor as claimedin claim 16, further comprising a layer of material between said firstelectrode and said second electrode for adsorbing said analytes in saidfluid.
 25. The sensor as claimed in claim 24, wherein said layer ofmaterial is selected from the group consisting of polymer, polymerinorganic material composite, Molecular Sieves, silica gel, alumina,porous carbon, and calcium carbonate particles, composite of palladiumparticles and inorganic materials, or composite of palladium particlesand polymer, polymer modified porous ceramic particles including porousalumina, or polymer modified porous silica, and polymer modified porousglass beads.
 26. The sensor as claimed in claim 16, further comprisingmeans for implementing a temperature programming technique.
 27. Thesensor as claimed in claim 16, further comprising a reference sensoridentical to said single analytical sensor for measuring AC relatedelectrical properties of background environment accompanied with onlysaid fluid.
 28. The sensor as claimed in claim 27, wherein saidbackground environment includes the group comprising temperaturevariation, polymer film aging effect and humidity level.
 29. The sensoras claimed in claim 16, wherein said AC related electrical propertiesare used for detection of said analytes present in said fluid.
 30. Thesensor as claimed in claim 16, further comprising odorants as saidanalytes.
 31. A sensor, comprising: a. a single analytical sensorcomprising two electrodes; b. said two electrodes are maintained in aclosely spaced apart relationship; c. means for connecting said singleanalytical sensor having said two electrodes to an alternating current(AC) analyzing device; d. a fluid containing analytes is positionedbetween said two electrodes, said fluid containing said analytesbridging said two electrodes; e. means for applying various frequenciesof AC signals to said fluid containing said analytes; f. means fordetecting AC related electrical properties of said fluid containing saidanalytes according to said applied various frequencies of AC signals,said electrical properties are detected at each of said appliedfrequencies; and g. said AC related electrical properties according toone of said applied frequencies is used for detection of said analytespresent in said fluid.
 32. The sensor as claimed in claim 31, whereinsaid AC related electrical properties include impedance of said fluidcontaining said analytes.
 33. The sensor as claimed in claim 31, whereinsaid AC related electrical properties include resistance of said fluidcontaining said analytes.
 34. The sensor as claimed in claim 31, whereinsaid AC related electrical properties include reactance of said fluidcontaining said analytes.
 35. The sensor as claimed in claim 31, whereinsaid AC related electrical properties include phase angle of said fluidcontaining said analytes.
 36. The sensor as claimed in claim 31, whereinsaid AC related electrical properties include voltage of said fluidcontaining said analytes.
 37. The sensor as claimed in claim 31, whereinsaid AC related electrical properties include current of said fluidcontaining said analytes.
 38. The sensor as claimed in claim 31, whereinsaid electrodes are made of electrically conductive materials.
 39. Thesensor as claimed in claim 31, further comprising a layer of materialbetween said two electrodes for adsorbing said analytes in said fluid.40. The sensor as claimed in claim 39, wherein said layer of material isselected from the group consisting of polymer, polymer inorganicmaterial composite, Molecular Sieves, silica gel, alumina, porouscarbon, and calcium carbonate particles, composite of palladiumparticles and inorganic materials, or composite of palladium particlesand polymer, polymer modified porous ceramic particles including porousalumina, or polymer modified porous silica, and polymer modified porousglass beads.
 41. The sensor as claimed in claim 31, further comprisingmeans for implementing a temperature programming technique.
 42. Thesensor as claimed in claim 31, further comprising a reference sensoridentical to said single analytical sensor for measuring AC relatedelectrical properties of background environment accompanied with onlysaid fluid.
 43. The sensor as claimed in claim 42, wherein saidbackground environment includes the group comprising temperaturevariation, polymer film aging effect and humidity level.
 44. The sensoras claimed in claim 31, further comprising odorants as said analytes.45. A sensor, comprising: a. a single analytical sensor comprising afirst electrode and a structural member functioning as a secondelectrode; b. said first electrode and said structural member aremaintained in a closely spaced apart relationship; c. means forconnecting said single analytical sensor having said first electrode andsaid structural member to an alternating current (AC) analyzing device;d. a fluid containing analytes is positioned between said firstelectrode and said structural member, said fluid containing saidanalytes bridging said first electrode and said structural member; e.means for applying various frequencies of AC signals to said fluidcontaining said analytes; f. means for detecting AC related electricalproperties of said fluid containing said analytes according to saidapplied various frequencies of AC signals, said electrical propertiesare detected at each of said applied frequencies; and g. said AC relatedelectrical properties according to one of said applied frequencies areused for detection of said analytes present in said fluid.
 46. Thesensor as claimed in claim 45, wherein said AC related electricalproperties include impedance of said fluid containing said analytes. 47.The sensor as claimed in claim 45, wherein said AC related electricalproperties include resistance of said fluid containing said analytes.48. The sensor as claimed in claim 45, wherein said AC relatedelectrical properties include reactance of said fluid containing saidanalytes.
 49. The sensor as claimed in claim 45, wherein said AC relatedelectrical properties include phase angle of said fluid containing saidanalytes.
 50. The sensor as claimed in claim 45, wherein said AC relatedelectrical properties include voltage of said fluid containing saidanalytes.
 51. The sensor as claimed in claim 45, wherein said AC relatedelectrical properties include current of said fluid containing saidanalytes.
 52. The sensor as claimed in claim 45, wherein said electrodesare made of electrically conductive materials.
 53. The sensor as claimedin claim 45, further comprising a layer of material between said firstelectrode and said second electrode for adsorbing said analytes in saidfluid.
 54. The sensor as claimed in claim 53, wherein said layer ofmaterial is selected from the group consisting of polymer, polymerinorganic material composite, Molecular Sieves, silica gel, alumina,porous carbon, and calcium carbonate particles, composite of palladiumparticles and inorganic materials, or composite of palladium particlesand polymer, polymer modified porous ceramic particles including porousalumina, or polymer modified porous silica, and polymer modified porousglass beads.
 55. The sensor as claimed in claim 45, further comprisingmeans for implementing a temperature programming technique.
 56. Thesensor as claimed in claim 45, further comprising a reference sensoridentical to said single analytical sensor for measuring AC relatedelectrical properties of background environment accompanied with onlysaid fluid.
 57. The sensor as claimed in claim 56, wherein saidbackground environment includes the group comprising temperaturevariation, polymer film aging effect and humidity level.
 58. The sensoras claimed in claim 45, further comprising odorants as said analytes.